Base price for National Grid shares from British Gas privatisation

I bought British Gas shares at privatisation in 1986, over the years they have diverged into shares in National Grid (22/5/2017), Centrica and Shell. I wish to sell the National Grid shares, can somebody please advise what do I use as the base price for these shares or suggest where I can find this information. Thanks in advance.

Hi,

You would need to check with the companies direct or contact the London Stock Exchange.

Both may charge a nominal fee for this.

Thank you.